1. Field of the Invention
The present invention relates to a method for manufacturing disk-shaped information carriers which can be optically read out, and more particularly to such carriers, manufactured of transparent synthetics, (for example polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), polycarbonate (PC), polysulfone (PS), etc.), that have a high storage density. The method utilizes a molding press equipped with at least one pressing matrix, and two molding halves movable with respect to one another during the molding cycle and also able to be heated and cooled. In addition, the molding halves are provided with a squeezing gap in order to limit the hollow space of the mold.
2. Description of the Prior Art
Molding methods for the manufacture of synthetic, disk-shaped information carriers are known, U.S. Pat. No. 3,039,139 can be seen in this regard. The development of information carriers having a very high storage density which can optically be read out have become known for video recordings and, more recently, for digital audio recordings as well. These information carriers place high demands on manufacturing methods using a pressing with respect to planarity and optical properties of the finished memory disks.
As has been shown, thermoplastic materials with a sufficiently low glazing or glass temperature can be employed for pressing such information carriers. However, given disks made out of the PVC/PVA copolymer which is usually employed, the dimensional stability is no longer guaranteed at higher temperatures. Disks of PMMA, which is only slightly more thermally stable, also have problems because of the excess moisture sensitivity of the material. Under certain conditions, this sensitivity of the material requires a sandwich construction of the video LP disk which consists of two disks bonded to one another at the information side. This construction is particularly cost-disadvantageous for the shorter informational programs which are to be accommodated on one disk side. In order to insure their planarity, both types of disks--even after storage, require more involved packaging than is standard in the recording field.
In processing thermoplastic materials having a high glass (or glazing) temperature, an improved dimensional stability can be achieved, avoiding the necessity for a sandwich structure and its required special packaging measures. However, with this material different difficulties arise in the manufacture of such disks with respect to uneveness, particularly with respect to optical double diffraction effects. In the optical read-out of the information impressed in the form of pits into a disk surface, such double diffraction effects result in an undesired modulation of the light source (normally a laser). This modulation disrupts the fault-free reproduction of the stored information.